In automotive engine controls it is important to determine accurate values of mass air flow in each engine operating condition by a method insensitive to manufacturing tolerances, aging and ambient variation in order to meet the goal of obtaining good fuel economy, low exhaust emissions and good vehicle performance.
Many systems and methods have been proposed for measuring the mass rate of air flow into an internal combustion engine. Generally these systems require an air flow sensing element positioned in the air stream to sense air flow. For example, some prior systems provide for a vane that is moved by the air flow to an angular position which is a measure of air flow. Other systems employ a constant temperature anemometer positioned in the air stream. The latter is more commonly used and is more accurate for high air flow rates than for low rates. It is desired, however, to have accuracy over the whole range of engine operation. To this end it is proposed to either supplement or replace the anemometer with a system which is accurate at low mass air flow rates as well as at high rates.
It is already known to measure mass air flow without a special air meter. For example, U.S. Pat. No. 4,446,523 to Reinke discloses an open loop system in which the mass air flow into the engine is determined by the measured throttle angular position, the sensed pressure above the throttle and a stored schedule of values dependent upon the ratio of the pressure below the throttle to the pressure above the throttle. U.S. Pat. No. 3,871,214 to Masaki et al shows an open loop system for measuring air flow at sonic flows only, and is coupled to a supplementary air flow measuring device for subsonic flow measurement. The sonic air flow is computed from measured pressure above and below the throttle and the throttle position. U.S. Pat. No. 4,750,352 discloses a mass air flow meter for an engine equipped with a bypass around the throttle with an adjustable valve for varying the air at idle, wherein the mass air flow is determined from measured throttle position, variable valve position, barometric pressure, manifold pressure and manifold temperature.